And that's the amazing thing about portobellos. Though they have a high water content, if you plunk them into a flavorful marinade, they still absorb it quickly.
I started by swapping out the beef in favor of that most steak-like of mushrooms, the portobello. Actually, it's just the roomy cap of the portobello, filled to the brim with roasted red peppers, grilled scallions, olives and mushroom trimmings, then topped with melted provolone cheese, and lubricated with a little bit of rosemary mayonnaise.
Maybe you'll want to grill and chop up some complementary mushrooms -- like shiitake or oyster -- and put them on top of the portobello. Maybe you'll opt to top it off with grilled broccoli, asparagus or onions.